docs: def _get_keycache(self, parentity, branch, turn, tick, *, forward):
          """Get a frozenset of keys that exist in the entity at the moment.
  
          With ``forward=True``, enable an optimization that copies old key sets
          forward and updates them.
  
          """
          lru_append(self.keycache, self._kc_lru, (parentity+(branch,), turn, tick), KEYCACHE_MAXSIZE)
          return self._get_keycachelike(
              self.keycache, self.keys, self._get_adds_dels,
              parentity, branch, turn, tick, forward=forward
          )